Energy independence is a modern priority driven both by anticipated climatic changes due to global warming and ever-increasing reliance on imported hydrocarbon derived energy sources. The need for renewable, clean and abundant energy sources is obvious from the likes of solar energy, especially from silicon photovoltaics (PVs), which is high on the list. A factor preventing the more rapid adoption of silicon PVs is their current high cost driven in part by limited supplies of the photovoltaic grade silicon (i.e., “Sipv”) used to manufacture monocrystalline or polycrystalline wafers.
Although silicon is the second most abundant element in the earth's surface, highly purified silicon needed to make PV cells is expensive because of the energy intensive, complex, and costly processes currently used in its manufacture. The large capital costs involved means that Sipv production takes years to ramp up. In 2006, the demand, by volume, for Sipv exceeded the demand for semiconductor or electronics grade silicon (i.e., “Sieg”) for the first time. The few companies manufacturing Sipv are currently unable to meet growing customer demand thereby reducing the potential for rapid growth in solar energy.
All grasses (wheat, rice, barley, oats, etc.) take up SiO2, depositing significant amounts in their stalks and seed hulls with minimal incorporation of the standard impurities found in “high purity quartz.” Thus the plants naturally pre-purify the silica incorporated in their structure. For example, rice hulls have the high silica contents for these grasses.
Many Sipv and Sieg manufacturers rely on what is known as the Siemens Si purification process (i.e., “the quartz rock purification process”) and modifications thereof. This process involves multiple steps starting from quartz rock and carbon sources to produce metallurgical grade Si (i.e., “Simg”). Simg is further processed to produce corrosive, toxic, and potentially polluting chlorosilanes that are then subject to several high-energy steps to produce Sieg/pv.
Currently a large majority of Si compounds may be produced from Simg, which may be produced by carbothermal reduction of silica (equation 1) in a relatively high temperature (e.g., greater than about 1500° C.), relatively high capital equipment and relatively high energy intensive process, (e.g., equations 1-3). The much higher purity required for electronic (Sieg) and photovoltaic (Sipv) grade silicon, requires additional processing steps as those typically utilized in the Siemens process (and related modifications), such as the steps described by equations (4)-(6). The reactions described by equations (4)-(6) will generate considerable SiCl4, HCl, Cl2 or any combination thereof as byproducts, which may have limited uses. It is further believed that it may be possible to recycle HCl, SiCl4, or both with little loss. Nonetheless, because chlorosilanes, Cl2 and HCl gases are corrosive, toxic, and polluting, the production processes require expensive and extensive environmental controls or other safeguards, which may add to the overall cost of the materials.SiO2+2C1700° C.>2CO+Simg  (equation 1)Simg+EtOHcatalyst>Si(OEt)4+H2  (equation 2)Simg+MeClCu/Sn catalyst/300° C.>MeSiCl3+Me2SiCl2(silicone/polysiloxane precursor)  (equation 3)Simg+HClCu/Sn catalyst/300° C.>HSiCl3 and/or SiCl4  (equation 4)4HSiCl3disproportionation catalyst>SiH4+3SiCl4  (equation 5)H2+HSiCl3(or SiH4)hot wire/rod>Sipv(or Sieg)+HCl (recycled)  (equation 6)
As such, there remains a need in the art to continue to find alternative ways to produce high purity silicon materials and derivatives thereof. One approach that has been followed is described in U.S. Pat. No. 4,214,920, (Exxon), filed Mar. 23, 1979, by Amick et al., which is herein incorporated by reference for all purposes. Exxon describes the use of rice hulls that are coked, but not to rice hull ash from a burning step (referred to herein as “RHA”), to produce Si directly with boron contents of ≦1 ppm. With reference to FIG. 1, this process includes a step of providing rice hulls 10, (referred to herein as “RH”). RH, regardless from where in the world it is obtained, has relatively similar impurity levels. The Exxon process includes a cleaning step 11, of leaching the RH in a boiling 10 percent aqueous solution of HCl followed by washing with electronics grade water. Thereafter, the process includes a step of coking 12 the rice hulls at 900° C. (in a non-oxidative atmosphere, with considerable evolution of gases and smoke) in flowing Ar/1 percent HCl to form a material with a C:SiO2 ratio of about 4:1 while preserving the low impurity contents. Following the coking step, the patent discloses a step of further coking 15 at about 950° C. with flowing CO2 to adjust the C:SiO2 ratio to about 2:1. The resulting particles are then converted into pellets using a step of pelletizing 16 that includes compounding the particles with a sucrose binder and then forming the mixture into pellets. The process also includes a step of heating 17 the pellets in an electric arc furnace (e.g., using an Ar atmosphere) to about 1900° C. to produce a photovoltaic grade of silicon.
As such, there is a need for a process for synthesizing one or more of a wide variety of Si compounds directly or indirectly from high surface area SiO2 derived from an agricultural waste products (“AWP”), such as rice hull ash, where the process is characterized as being simpler (e.g., requires fewer steps, such as fewer heating steps, fewer washing steps, fewer leaching steps; uses one or more steps that are relatively fast; uses a relatively low processing temperature, or any combination thereof), more environmentally friendly, more energy efficient (e.g., at least 20 percent more energy efficient), or any combination thereof.